The present invention pertains to a fiber mat, especially one which consists of a material resistant to heat and alternating thermal stress and is especially suitable for holding ceramic monoliths in exhaust systems of internal combustion engines, e.g., engines of motor vehicles, which said exhaust systems are subject to pulsating load.
In modern motor vehicles, the exhaust gases of the internal combustion engines used to drive the vehicles are regularly subjected to catalytic aftertreatment in order to convert especially toxic components of the exhaust gases into less toxic substances. The catalytic converter devices typical for this comprise a ceramic monolith, which has a large number of narrow exhaust gas channels, whose walls are coated with a catalytically active material, usually platinum, palladium or rhodium, so that the exhaust gas passing through these channels can interact practically completely with the catalyst material.
The ceramic monolith or the ceramic monoliths are usually accommodated in metal housings, wherein the particular ceramic monolith is mounted by means of a more or less markedly flexible mat, which is arranged within an annular gap between an outer circumferential surface of the ceramic monolith and an inner wall of the housing and fixes the monolith essentially by friction. Moreover, this mounting mat shall compensate inevitable manufacturing tolerances of the ceramic monolith and the metal housing as well as the different coefficients of thermal expansion of the housing and the monolith. The mounting mat may be subject to high alternating stresses partly because the exhaust gas temperatures reach extraordinarily high values compared with the normal temperature of the atmosphere and partly because extreme cooling of the housing compared with the ceramic monolith may occur due to splash water on roads wet from rain or in car washes. In addition, the exhaust gas flow is characterized by pronounced pulsations, which induce vibrations of the wall of the metal housing and continuously subject the ceramic monolith to pulsations on the side on which the exhaust gas enters.
Mounting mats for ceramic monoliths in exhaust systems are currently designed as xe2x80x9cvery irregularxe2x80x9d knitted wire fabrics and/or as so-called expanded mats, which may contain mica particles expanding under heat and ceramic components, especially in the form of fibers.
The primary object of the present invention is to provide fiber mats suitable for mounting ceramic monoliths in exhaust systems with especially high load-bearing capacity and pronounced long-term elasticity.
This object is accomplished according to the present invention by the fibers being arranged in ordered layers, such that the fibers of one layer extend at right angles to the fibers of an adjacent layer or layers. Each fiber of an intermediate layer located between two adjacent layers lies on the fibers of each adjacent layer at a plurality of contact points. The distances between the contact points are, on average, greater than the fiber diameter, wherein the contact points of the fibers with the fibers of one adjacent layer are located predominantly offset from the contact points with the fibers of the other adjacent layer.
The present invention is based on the general idea of designing the fiber mat as a swath of fibers with predominantly ordered fibers, such that two fibers of adjacent layers, which fibers are in contact with one another, lie on a fiber or fibers of the other layers only at a certain distance on their sides facing away from the contact point in question. This ultimately means that contact points and free spaces follow one another alternatingly more or less frequently in the direction of the thickness of the mat, i.e., at right angles to the plane of the mat. Thus, a fiber in a contact point can regularly yield in the direction of a free space located diametrically opposed to the contact point relative to the fiber axis, so that the contact points are located at least predominantly on fiber areas that yield during load under bending. These fibers areas consequently resemble flexible xe2x80x9cbeams,xe2x80x9d which are mounted on both sides of a contact point loaded in one direction for support against this load.
According to a preferred embodiment of the present invention, the fiber diameter is between approx. 3 xcexcm and 15 xcexcm, the fiber diameter of at least 5 xcexcm being preferred in order to avoid irritations of the human skin if small fragments are possibly formed and, moreover, to prevent the fiber fragments from being able to be inhaled at a high probability. In the case of mounting mats for ceramic monoliths, vitreous or ceramic metal oxides are preferred as fiber materials because of their high thermal loadability. Even though these are usually extremely brittle materials, high long-lasting elasticity can be guaranteed in the direction of the thickness of the mat because of the fiber swath provided according to the present invention as well as the small fiber diameter.
At least part of the fiber layers may optionally consist of wave-shaped or coiled fibers. As a result, it is possible, on the one hand, to increase the distances between the support points on a fiber. On the other hand, it can be guaranteed relatively easily with such fibers that free spaces, which can be utilized for the bending movements of fibers of adjacent layers, will regularly remain within a fiber layer.
The fibers of the fiber swath are bent in a more or less wave-shaped pattern under pressing load. This leads per se to a corresponding tensile load of the fibers and/or to a shortening of the mat in the longitudinal and transverse directions. This effect can be compensated by the fibers of the mat having a, preferably irregular, wavy shape with a relatively large space wave length and comparatively small space amplitude in the unloaded and unstressed state. The fibers can thus xe2x80x9cstretchxe2x80x9d somewhat without being subjected to excessive tensile load during the pressing of the mat.
The order of the fiber swath of the mat can be fixed before installation or insertion by bonding and/or by means of auxiliary threads. It is thus possible, in particular, to keep the fiber layers under pretension in the direction of the thickness of the mat by means of auxiliary threads or by the bonding. The insertion or the installation of the mat can be markedly facilitated with such measures.
If the auxiliary threads or the bonding consist of a material which dissolves during heating (or under the effect of other measures), the pressing forces generated by the mat in a gap after the insertion can be increased by the dissolution of the auxiliary threads or the bonding.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.